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322            MOEPHOLOGY AND EVOLUTION
Mammals as the result of cerebral changes, which Professor Elliot Smith described as follows: " In the Primates it was the precocious expansion of the brain and the simultaneous cultivation of the visual, auditory, tactile and motor areas of the cerebral cortex that first differentiated the earliest Primates from all other Mammals, and provided them with the germs of the capabilities and the means of attaining the supreme position expressed in the name of the Order."
The fossil Prosimiae of the American Eocene belong to two Families, the Notharctidse, which were Lemur* oidea, and the Anaptomorphjdse, which were Tarsioidea.
The Family Motharctidse includes the genus Notharctus, with several species, and the genus Pely-codus, with one species. And the remains, which include a nearly complete skull, a skeleton, fragments-of jaws and teeth, and some limb bones, show that the animals varied considerably in size. The oldest forms-were quite small, whereas Notharctus crassus was as large as a Howling Monkey (Gregory). Many examples might be taken from the different Mammalian Orders to show that there is a progressive increase in the size of the body in the course of evolution. The general characters of the skeleton resemble those of the modern Lemurs (see p. 17), but the cranial cavity was smaller, and the teeth were more primitive. The variations in the dental formula are.
I. Ancestral placental Mammals   I|C^PM|Mf II. Notharctidse.........    If CJPMJHf
III. Modern Lemurs       ......    If C-J PMf Mf
Besides these quantitative changes there have been specialisations of more or less profound character in most